The present invention relates generally to staged energy systems and particularly to analog staged energy systems with fault tolerant and transparent dynamic load distribution based on stage status and runtime. The present invention will be explained with reference to a boiler system that includes multiple boilers, however it will be understood that the invention is applicable to a variety of energy systems.
The design of boiler systems for commercial, industrial, and institutional buildings is typically performed by a consulting engineer, who specifies the type, number, and size of boilers needed for heating systems. There are many factors that weigh into the decisions an engineer makes when selecting and sizing boilers for a heating system including capacity of the system, what is the load present on the system, and what is the worst case load conditions that would be required for the system to provide adequate heat. If there is not adequate heat available for a building, the notion of occupant comfort relating to heating and cooling could be compromised on a grand scale, including the entire building being serviced by the boiler heating system.
The specification of a single, large heating capacity boiler can satisfy the heating demand for the worst load conditions, which in cold climate Heating Ventilating and Air Conditioning (HVAC) applications would be defined as the xe2x80x9cdesign temperaturexe2x80x9d. Typically, a very cold outside air temperature requires the full capacity of the boiler to provide heat for the building. However, the typical use of this load level would be limited to a total of less than 2% of the total year time. Design of smaller, but multiple boiler system can lead to a reduction of the xe2x80x9cexcess capacityxe2x80x9d of the boiler system on a typical system from 40% to 4%, which represents significant operational savings, increased system efficiency, and improved heat system reliability. For example a lightly loaded system could have its requirements met by using only 1 of 3 smaller, more efficient boilers instead of using ⅓ the capacity of a larger boiler.
The control system for a multiple boiler or staged boiler system is necessarily different from the control system for a single boiler. For example, in a multiple boiler system, consideration must be given to the number of stages, whether the boilers have variable firing rates, under what conditions an individual boiler will be turned on or turned off, the strategy for equalizing run time of the individual boilers, what occurs in the event of the failure of an individual boiler and other factors. In the past these considerations have frequently required a more or less custom design and installation process and the increased costs that accompany such a process.
Flame safety boiler controls directly affect elements that may cause an unsafe condition. Flame safety controls have very high safety standards, and require strict testing and failure analysis. Manufacturers of flame safety products typically provide flame safety controllers to an original equipment manufacturer (OEM) for boilers. The OEM then integrates these controls into their boiler designs. The use of staged boiler systems doe not affect the flame safety controls which are fully operational when staged boiler control systems are utilized.
Thus there is a need for a boiler control system that takes into consideration the number of boiler stages and whether the boilers have a variable firing rate, provides a technique for decisions as to firing rate or adding or deleting a boiler, equalizes run times and automatically compensates in the event of failure of an individual boiler.
Thus a need exists for a low-cost high-performance sequencing controller for use with multiple boiler systems and other multiple unit energy systems.
The present invention solves these and other needs by providing a method for controlling energy systems such as multiple boiler systems to meet an energy need. In one aspect a controller is configured as a sequencer and the remaining controllers act as individual boiler controllers. The energy need is determined by measurements at the sequencer. Individual boiler controllers periodically send status messages to the sequencer and a record of runtimes of the boilers is maintained at the sequencer. The sequencer periodically sends control commands to the boiler controllers to add or delete boilers. The control commands give consideration to the runtimes of the boilers and the efficiency of operating an alternate number of boilers.